The present invention relates generally to radio communication systems and, in particular, to a miniature printed spiral antenna which can be incorporated into a portable terminal.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next generation technology, e.g., time division multiple access (TDMA) or code division multiple access (CDMA). Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure.
To provide an acceptable level of equipment compatibility, standards have been created in various regions of the world. For example, analog standards such as AMPS (Advanced Mobile Phone System), NMT (Nordic Mobile Telephone) and ETACS and digital standards such as D-AMPS (e.g., as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications adopted by ETSI) have been promulgated to standardize design criteria for radio communication systems. Once created, these standards tend to be reused in the same or similar form, to specify additional systems. For example, in addition to the original GSM system, there also exists the DCS1800 (specified by ETSI) and PCS1900 (specified by JTC in J-STD-007), both of which are based on GSM.
However, the most recent evolution in cellular communication services involves the adoption of additional frequency bands for use in handling mobile communications, e.g., for Personal Communication Services (PCS) services. Taking the U.S. as an example, the Cellular hyperband is assigned two frequency bands (commonly referred to as the A frequency band and the B frequency band) for carrying and controlling communications in the 800 MHz region. The PCS hyperband, on the other hand, is specified in the United States to include six different frequency bands (A, B, C, D, E and F) in the 1900 MHz region. Thus, eight frequency bands are now available in any given service area of the U.S. to facilitate communication services. Certain standards have been approved for the PCS hyperband (e.g., PCS1900 (J-STD-007)), while others have been approved for the Cellular hyperband (e.g., D-AMPS (IS-136)).
Each one of the frequency bands specified for the Cellular and PCS hyperbands is allocated a plurality of traffic channels and at least one access or control channel. The control channel is used to control or supervise the operation of mobile stations by means of information transmitted to and received from the mobile stations. Such information may include incoming call signals, outgoing call signals, page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, hand-off, and cell selection or reselection instructions as a mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control and voice channels may operate using either analog modulation or digital modulation.
The signals transmitted by a base station in the downlink over the traffic and control channels are received by mobile or portable terminals, each of which have at least one antenna. Historically, portable terminals have employed a number of different types of antennas to receive and transmit signals over the air interface. For example, monopole antennas mounted perpendicularly to a conducting surface have been found to provide good radiation characteristics, desirable drive point impedances and relatively simple construction. Monopole antennas can be created in various physical forms. For example, rod or whip antennas have frequently been used in conjunction with portable terminals. For high frequency applications where an antenna""s length is to be minimized, another choice is the helical antenna.
Presently, antennas for radio communication devices, such as mobile phones, are mounted directly on the phone chassis. However, as the size and weight of portable terminals continue to decrease, the above-described antennas become less advantageous due to their size. As a result, built-in antennas will be necessary for these future compact portable terminals which are capable of operating in a 300 MHz to 3000 MHz frequency range.
Conventional built-in antennas currently in use in mobile phones include microstrip patch antennas and planar inverted-F antennas. Microstrip antennas are small in size and light in weight. The planar inverted-F antenna (PIFA) has already been implemented in a mobile phone handset, as described by K. Qassim, xe2x80x9cInverted-F Antenna for Portable Handsetsxe2x80x9d, IEE Colloqium on Microwave Filters and Antennas for Personal Communication Systems, pp.3/1-3/16, February 1994, London, UK. And, more recently, Lai et al. has published a meandering inverted-F antenna (WO 96/27219). This antenna has a size which is about 40% of that of the conventional PIFA antenna.
FIGS. 1A and 1B illustrate the conventional planar patch antenna compared to the meandering inverted-F antenna described in Lai et al. The conventional planar patch antenna of FIG. 1A has both a size and length equal to, for example, a quarter wavelength of the frequency to which the antenna is to be made resonant. The conventional planar patch antenna also has a width of W. The meandering inverted-F antenna, illustrated in FIG. 1B, also has a length equal to a quarter wavelength of the resonant frequency and a width equal to W; however, the size of the meandering inverted-F antenna is reduced to about 40% of the size of the conventional planar patch antenna. This reduction in size is attributable to the antenna""s meandering shape.
As mobile phones become smaller and smaller, both conventional microstrip patch and PIFA antennas are still too large to fit the future small phone chassis. This is particularly problematic when next generation phones need multiple antennas for cellular, wireless local area network, GPS and diversity.
The present invention seeks to overcome the above-identified deficiencies in the art by providing a built-in printed spiral antenna which is small enough to satisfy the needs of future compact mobile terminals. According to exemplary embodiments, a built-in antenna is provided which includes a printed spiral metal strip that is connected to the mobile terminal""s printed circuit board via a substrate. Matching of the antenna is performed by a matching bridge which is positioned between a feeding pin and a grounded post. By adjusting the length of the matching bridge, the matching of the antenna can be changed. In an alternative embodiment, a loading resistor is attached to the matching bridge in order to enhance the bandwidth of the antenna. The size of the antenna of the present invention can be reduced to 20-30% of the conventional PIFA antenna (i.e., less than {fraction (1/10)} of the wavelength of the operating frequency). As a result, the antenna can be used in a very compact chassis.